1. Field of the Invention
The invention relates to a method and apparatus for equalizing the power of at least one frequency in a multi-wavelength optical signal, and limiting the power in an optical system. More particularly, the invention relates to a method and apparatus for equalizing the power of at least one frequency in a multi-wavelength optical signal that equalizes without spectrally dispersing the signal.
2. Background of the Related Art
Extremely low losses in optical fibers have made them the transmission medium of choice for communications networks. These losses, however, are not zero so transmitted optical signals need to be amplified to compensate for the losses. Moreover, signals in all but the simplest networks split among several paths would each require amplification.
In a few short years, the erbium doped fiber amplifier (EDFA) has revolutionized optical networks by providing amplification of optical signals without conversion to and from the electrical domain. FIG. 1 shows the gain versus input signal wavelength for 3 different EDFAs based on A1/P Silica, Silicate L22, and Fluorozirconate F88, respectively. FIG. 1 is reproduced from J. Miniscalco, J. Lightwave Tech. 9, 234 (1991), which is hereby incorporated by reference herein. As indicted by FIG. 1, each type of EDFA has a gain peak centered at about 1530 nm (nanometers), with substantially reduced gain at wavelengths below about 1490 nm and above about 1580 nm. For an input signal between 1490 nm and 1580 nm, the gain of the EDFA is still strongly dependant on input wavelength. For virtually any given multi-wavelength input signal, the EDFA will amplify each wavelength a different amount. FIG. 1 shows that even when the multi-wavelength input signal has an even power distribution, the output of the EDFA will not have a uniform distribution of power between the wavelengths.
When amplifying a single wavelength, the pump power and the signal wavelength determine the gain. To maintain a substantially constant optical power level in a conventional substantially single wavelength system throughout the system, each EDFA has a feedback circuit that controls the pump power while monitoring the output power. Thus, even a relatively simple system relying on a single frequency requires a complex feedback mechanism to assure a stable network.
The EDFA has a more complex behavior when it simultaneously amplifies several wavelengths. The gain at a particular wavelength depends, in addition to the pump power and the wavelength, on its input power relative to those at the other wavelengths. Specifically, the gain is greatest for the wavelengths that already have the most power and least for the weakest power wavelengths. The gain differential is modest, so it is generally not a serious problem for small multi-wavelength networks containing just a few EDFA""s. But as the size of the network grows, the effects of the gain differential accumulate, and can lead to instability in the network with a few wavelengths dominating.
The output spectrum of the EDFA ideally should be flat, or equalized, to avoid instabilities due to single wavelength domination. It is not sufficient to simply equalize the EDFA gain. The instability still occurs with a flat gain if the input spectrum is not flat. Thus, a general solution to this problem must equalize the EDFA output rather than its gain.
If it is possible to predict with reasonable accuracy the input power distribution to each EDFA, then one can insert compensating filters that attenuate the stronger wavelengths more than the weaker ones so that each EDFA output power distribution is spectrally flat or equalized. For example, see C. R. Giles and D. I. DeGiovanni, xe2x80x9cDynamic Gain Equalization in Two-Stage Fiber Amplifiers,xe2x80x9d IEEE Phot. Technol. Letts., 2,866-868, (1990); M. Tachibana, R. I. Laming, P. R. Morkel and D. N. Payne, xe2x80x9cErbium-Doped Fiber: Amplifier with Flattened Gain Spectrum,xe2x80x9d IEEE Phot. Technol. Letts., 3, 118-120, (1991); and A. E. Willner and S. -M. Hwang, xe2x80x9cTransmission of Many WDM Channels Through a Cascade of EDFA""s in Long Distance Links and Ring Networks,xe2x80x9d J. Lightwave Technol., 5, 802-816, (1995), which are hereby incorporated by reference. But such an approach works best if the network is static. A change in the input power distribution of one EDFA will disrupt power equalization throughout the system. In turn, this will upset the input power distribution to other EDFA""s, disrupt their equalization and may eventually cause a network-wide instability. Such changes in the power distribution would be common in reconfigurable systems where wavelengths are intentionally switched from one path to another.
It is these reconfigurable optical fiber systems that are most attractive for large multi-wavelength networks. For example, see G. K. Chang, G. Ellinas, J. K. Gamelin, M. z. Iqbal and C. A. Brackett, xe2x80x9cMultiwavelength Reconfigurable WDM/ATM/SONET Network Testbed,xe2x80x9d J. Lightwave Technol., 14, 1320-1340, (1996); and R. E. Wagner, R. C. Alfemess, A. A. M. Saleh and M. S. Goodman, xe2x80x9cMONET: Multiwavelength Optical Networking,xe2x80x9d J. Lightwave Technol., 14, 1349-1355, (1996), which are hereby incorporated by reference. For these networks to function properly, the output of each EDFA must remain equalized even as the input power distribution varies. The gain required to maintain a flat output spectrum will vary as the input varies. A power equalizer must continuously sense the power at each wavelength and alter the power at that wavelength accordingly. The equalizer must treat each wavelength independently so that a strong wavelength will be attenuated without attenuating the weaker ones. Besides these basic requirements, the equalizer should have features that would make it attractive for widespread use such as scalability in the number of wavelengths, low cost, reliability, ease of use, etc.
Several approaches have been demonstrated to try to solve this problem. Experiments showing that the coupling between wavelengths in EDFA""s decreases at cryogenic temperatures because the gain becomes inhomogeneously broadened, suggest operating the EDFA at cryogenic temperatures. For example, see L. Eskildsen, E. Goldstein, V. da Silva, M. Andrejco and Y. Silberberg, xe2x80x9cOptical Power Equalization for Multiwavelength Fiber-Amplifier Cascades Using Periodic Inhomogeneous Broadening,xe2x80x9d IEEE Phot. Technol. Letts., 5, 1188-1190, (1993), which is hereby incorporated by reference. However, cryogenic cooling is not economically attractive.
A different approach is to disperse the light spectrally, measure the power at each wavelength, and adjust a tunable filter for each wavelength according to the measured powers. For example, see K. Inoue, T. Kominato and H. Toba, xe2x80x9cTunable Gain Equalization Using a Mach-Zehnder Optical Filter in Multistage Fiber Amplifiers,xe2x80x9d IEEE Phot. Technol. Letts., 3, 718-720, (1991); F. Su, R. Olshansky, G. Joyce, D. A. Smith and J. E. Baran, xe2x80x9cGain Equalization in Multiwavelength Lightwave Systems Using Acoustooptic Tunable Filters,xe2x80x9d IEEE Phot. Technol. Letts., 4,269-271, (1992); and F. Khaleghi, M. Kavehrad and C. Bamard, xe2x80x9cTunable Coherent Optical Transversal EDFA Gain Equalization,xe2x80x9d J. Lightwave Technol., 13,581-587, (1995), which are hereby incorporated by example. This approach requires a large amount of hardware, including a spectrometer, to be attached to each EDFA.
An example as disclosed in U.S. Pat. No. 5,155,780 to Zirngible, of a method to equalize the optical power in a network is an optical limiting amplifier. In the optical limiting amplifier, the input optical signal is divided into two signals. One signal is the input signal of the optical amplifier. The other signal is passed through a saturable absorber. The signal from the saturable absorber is fed back into the optical amplifier via the output of the optical amplifier. The output signal from the saturable absorber varies at a rate which is greater than the variation between the input signal and the saturable absorber, and thus forms a negative feedback loop to the optical amplifier.
For the above systems incorporating a saturable absorber, as the optical signal into the amplifier increases, the feedback signal from the saturable absorber increases at a faster rate. As the feedback signal increases in intensity, the gain saturation of the optical amplifier is increased, and the gain of the amplifier is proportionally reduced. Thus, the amplifier has an output of constant power regardless of the input strength of the input signal.
However, this method of achieving constant power from an optical amplifier has the drawback of requiring a negative feedback signal. The negative feedback signal adds complexity to the system, and requires directional couplers to be inserted into the system. This method of power equalization also has a drawback that it is limited to functioning at a single wavelength.
Another device as disclosed in U.S. Pat. No. 6,104,848 to Loyohara, et al., which can equalize optical signals consists of multiple light sources each generating its own wavelength signal, and each with its own amplifier. The outputs of each amplifier are multiplexed into an optical transmission line and a small portion of the signal is coupled from the transmission line to form a feedback loop. In the feedback loop, the power at each frequency in the optical signal is measured separately, and the measured power level of each signal controls the gain of that signal""s respective amplifier. A control unit then functions to separately adjust the gain of each amplifier at each light source so that the power at each frequency in the optical signal is equalized. One drawback of this system is that each frequency in the optical signal must be measured and adjusted separately, which requires a complicated system.
Another example, as disclosed in U.S. Pat. No. 5,812,710 to Sugaya, of a device that equalizes two signals of a multi-wavelength signal consists of a variable optical attenuator, an optical fiber amplifier, a light source to pump the amplifier, and a control unit which receives a small portion of the output signal from the amplifier. The control unit controls both the attenuating unit and the power of the pump light source. The control unit receives a feedback signal from the output of the optical amplifier and measures the intensity of the two wavelengths to be equalized. The control unit then adjusts the attenuation of the variable optical attenuator, and adjusts the power of the pump light source driving the optical amplifier. With suitable adjustments to the attenuator and pump light, the power of the two wavelengths can be equalized. The drawbacks with this system include the inability to equalize more than two wavelengths, and the need for a feedback signal from the output of the amplifier.
An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.
Another object of the invention is to provide methods and apparatus to equalize the power of at least one wavelength (or frequency) in a multi-wavelength optical signal.
In order to achieve at least the above-described objects of the present invention in whole or in part, there is provided an optical power equalizer including a filter with separably variable wavelength dependent transmission coefficients, wherein each coefficient decreases with increasing power for each respective wavelength coupled or input to the equalizer. Thus, the highest power wavelength output from an EDFA will be filtered more than the lower power wavelengths, making the output power from the EDFA more evenly distributed among the wavelengths. An example of a device with this power-dependent transmission function is a power equalizer based upon the photo-refractive effect. Such an equalizer can be placed downstream from each EDFA after an optical isolator without destabilizing the optical network so that no changes need to be made to the EDFA or to the other components in the system. By placing the equalizer after the isolator, reflections from the equalizer will not degrade the EDFA. The equalizer is not sensitive to reflections from down stream, so additional isolators are not necessary.
To further achieve the above-described objects of the present invention in a whole or in parts, there is provided an apparatus including an optical cavity adapted to receive and transmit an optical signal with a photo-reactive medium disposed within the optical cavity, wherein the photo-reactive medium is configured to reversibly form a diffraction grating adapted to scatter at least a portion of the optical signal in proportion to the intensity of the optical signal.
To further achieve the above-described objects of the present invention in a whole or in parts, there is provided a power equalizer including an optical cavity adapted to receive an optical signal containing a plurality of wavelengths; and a photo-refractive medium disposed within said optical cavity, wherein the optical cavity is configured to filter higher power wavelengths more than lower power wavelengths, thereby substantially equalizing power of at least a portion of said plurality of wavelengths.
To further achieve the above-described objects of the present invention in a whole or in parts, there is provided an apparatus to equalize power in an optical signal including an optical cavity for receiving an optical signal having at least one wavelength, wherein the optical cavity has a finesse optically matched to the optical signal, and a photo-reactive medium disposed within the optical cavity, wherein the photo-reactive medium and the optical cavity are configured to form a diffraction grating in response to the optical signal which reduces the intensity of at least one wavelength in the optical signal.
To further achieve the above-described objects of the present invention in a whole or in parts, there is provided a method for equalizing the power of a multi-wavelength optical signal including the steps of directing an optical signal into a reflective cavity containing a photo-reactive material, reversibly forming a diffraction grating within the reflective cavity, and scattering at least one frequency of the optical signal by passing the optical signal through the reversibly formed diffraction grating.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.